Field of Invention
The present invention relates to an electrostatic clamp and to a method of making an electrostatic clamp. The electrostatic clamp may form part of a lithographic apparatus.
Background Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Electrostatic clamps may be used in lithographic apparatus in which it is not practical to use a vacuum clamp. For example, some regions of a EUV lithographic apparatus operate under vacuum conditions, and consequently it may not be practical to use a vacuum clamp in those regions. An electrostatic clamp may be used instead. An electrostatic clamp may be provided to electrostatically clamp (i.e., hold) an object, such as a mask or a substrate (wafer) to an object support, such as a mask table or a wafer table, respectively. Such a clamp may be described as an object holder. Alternatively and/or additionally, the clamp may form part of an object holder. The object holder might sometimes be described, alternatively and/or additionally, as an object support.
It is desirable to provide, for example, an improved electrostatic clamp which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.